OK, I'll have a go......
There is a question (or 2) behind the original question of wether UK immigration is too high. Is the population of the UK too high? Is the future population of the UK too high? Will immigration provide the needs of the UK now and in the future? ...... etc, etc

Firstly as has been previously noted effective immigration is the amount of people coming into the country in exess of those emmigrating.
The future population of the UK is dependant on the base population birthrate pluss the immigrant birthrate. UK base population birthrate is not sustainable! By this I mean the base population death rate is higher than birthrate, ergo we need immigration to hold our population at the present levels. I have seen few people advocating a reduction in our population. This leaves us with argueing over the actual amount of immmigration we collectivly want!
If you think about it there is a natural selection involved in emmigration so that we are more likely to loose a highly qualified person than a lower one, if only because they are more likely to be financially able to relocate abroad. This slews the natural ballance of the population (allbeit slightly).
The opposite side of the equation is the earning potential of immigrants is greater than in their homeland, otherwise they would not be coming here!
If we take the long view here, the educational chances of an immigrants children are increased. This has a dramatic effect on the number of their children (there is a direct corolation here)!

So we need some net immigration, and the total number of immigrants (and their families) will initially increase but then slowly reduce to base levels) If anyone doubts this look at the birthrate in countries like Ireland over the last century.

Again this is a simplification of the complex nature of population change and has also been noted already we are being asked to apply an emotional answer rather than an impirical one!